Here's what I see as the main problem: You're dealing with at least one different library per file type, not to mention dealing with the individual functions required by those filetypes. In other words, excel is going to have different functions and requirements than word, both of which you'll need to accommodate and write for. If you're still dead set on writing this thing yourself I would pick one file type to deal with (.docx, .xlsx, etc) and get that working with a library like PHPExcel, and then start adding in more filetypes for that particular piece.
Or, you know, you could use google docs :)